They're both correct. In the first one, you're setting the time for when she was talking, so the past tense is reported speech. In the second, the time is in the future, so the present tense is fine.
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John LawlerFeb 17 '13 at 5:42

@Anonymous: Speaking for myself, I'm glad you didn't post it on ELL. Quite apart from the question of whether "when" associates with what she actually said, or the time at which she said it, I'm having a bit of trouble myself deciding which of the 4 permutations (comes/came, reported before/after she returns) are credible.
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FumbleFingersFeb 17 '13 at 23:21

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@FumbleFingers: Notice that I haven't taken a stand either way (and don't plan to). It wasn't my recommendation initially; I was merely clarifying Kris' comment. My follow-on comment simply makes it clear that native speakers shouldn't feel overqualified to ask a question on ELL.
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J.R.Feb 17 '13 at 23:48

2 Answers
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Her actual words were ‘I’ll do it when I come home’, and she meant that she’d fulfil some obligation or other on her return. When someone else says what she said, it becomes, in a rather formal style, ‘She said she would do it when she came home.’

‘She said she’d do it when she comes home’ says the same thing in a rather less formal way. Even less formal is ‘She said she’ll do it when she comes home.’

Of course, given OP's reported speech #1, it's equally possible her actual words were just "I'll do it", said at the time of coming home.
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FumbleFingersFeb 17 '13 at 17:31

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@Barrie,Anonymous: Putting aside the informality of she'd, I don't really see any formal/informal distinction between when she comes/came home. Surely the only difference is that with comes, we know for sure she hasn't got home yet, whereas you could use came before or after she returned.
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FumbleFingersFeb 17 '13 at 23:07

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...personally, I'm sure I'd always use comes if she's not yet back, and came afterwards. And I'm pretty sure no-one would use comes after she was back, but I'm not really sure whether anyone would use came before she'd returned (unless they didn't actually know whether she was back or not).
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FumbleFingersFeb 17 '13 at 23:17

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@FumbleFingers Just to add one more twist: ‘She said she’d do it when she comes home’ might mean she engaged to [make her bed, do the breakfast washing-up, whatever] when she comes home from school every day, rather than before departing for school. AND ‘She said she’d do it when she came home—but then she never came home, so it’s still undone.’
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StoneyBFeb 17 '13 at 23:22

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@StoneyB: The unresolved question for me is: would anyone use came (if the intended sense is that's when she'll do it), if they know perfectly well that she's not yet back? I now think I would even say that myself, if I had reason to suspect she either wasn't ever coming home, or that she wouldn't honour her undertaking for some other reason.
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FumbleFingersFeb 17 '13 at 23:35

Yes, both are considered correct, it really just depends on what you want to say.

In (1) the action is presumed to be 100 % complete, and it is in the past. However, the sentence is a bit ambiguous and leaves the possibility open to the idea that the action may still not be done.

In (2) the statement was made in the past, but it refers to a future event by using the present tense. The present tense is pretty useful, because it can also refer to future events, like in your example.

I don't agree there's any reason whatsoever to presume the "coming home" in #1 is "in the past". It's the construction that's 100% ambiguous; both interpretations are equally valid, and with no other context, equally likely.
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FumbleFingersFeb 17 '13 at 17:28

@FumbleFingers and Patrick: Thank you for your responses! It's quite interesting to see how such a simple statement can be interpreted in so many differing ways by so many qualified people!
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AnonymousFeb 17 '13 at 22:58